Metronidazole: what it treats, how to take it, side effects
Last updated September 3, 2026.
Metronidazole is a nitroimidazole antimicrobial - it stops the growth of bacteria and certain parasites. You may know it as Flagyl. It treats a wide range of infections, and it enforces the most famous alcohol rule in medicine: none during treatment, and none for 3 days after.
What does it treat?
Infections of the reproductive system, gastrointestinal tract, skin, heart, bone, joint, lung, blood, and nervous system; certain sexually transmitted diseases; and bacterial vaginosis in women. It does not work for colds, flu, or other viral infections.
How do you take it?
Exactly as directed - the schedule varies: sometimes a single one-time dose, sometimes two to four times daily for up to 10 days or longer. Shake the suspension before each use and measure with a dosing spoon, syringe, or cup, not a kitchen spoon. Swallow extended-release tablets whole - never split, chew, or crush. Finish the full course even when you feel well. And the alcohol rule, stated plainly: no alcoholic beverages, and no products containing alcohol or propylene glycol, during treatment and for at least 3 days after your final dose - the combination causes nausea, vomiting, stomach cramps, headache, sweating, and facial flushing. Missed dose: take it when you remember unless the next one is close; never double up.
Side effects to know
- Common: vomiting, nausea, diarrhea, constipation, upset stomach, stomach cramps, loss of appetite, headache, dry mouth, a sharp metallic taste, and a furry-feeling or irritated tongue.
- Serious - call your doctor immediately: numbness, pain, burning, or tingling in your hands or feet, and seizures.
Who should not take it?
Anyone taking disulfiram, or who stopped it within the past 2 weeks - that combination is ruled out. Cockayne syndrome rules it out. Flag Crohn's disease, blood problems, kidney or liver disease, a yeast infection, and any condition affecting the brain. Pregnancy: women generally should not take metronidazole in the first 3 months; if you become pregnant, call your doctor. Breastfeeding: your doctor may tell you to pump and discard milk during treatment and for 48 hours after the final dose. Cimetidine interacts. And check your other products for hidden alcohol and propylene glycol - some mouthwashes and liquid medicines carry them.
When is it an emergency?
A seizure is a 911 call. Numbness or tingling in the hands or feet means calling your doctor immediately - nerve effects on this drug are not to be waited out. For an overdose, call Poison Control at 1-800-222-1222. Pymander's escalation routing is built and tested specifically for this class of decision; see the safety architecture working paper.
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Common questions
Why can't I drink alcohol on metronidazole?
The reaction is immediate and miserable. MedlinePlus says no alcoholic beverages - and no products containing alcohol or propylene glycol - during treatment and for at least 3 days after your final dose: the combination causes nausea, vomiting, stomach cramps, headache, sweating, and facial flushing. The 3-day tail matters because the drug clears slowly. Check mouthwash and liquid medicines too - some carry alcohol or propylene glycol.
Why does metronidazole leave a metallic taste?
A known, harmless quirk. MedlinePlus lists a sharp, unpleasant metallic taste - plus a furry-feeling tongue and dry mouth - among the common effects. It fades after the course ends. Water, sugar-free mints, and taking the doses with food are the usual workarounds.
Can I take metronidazole with disulfiram?
Never together. MedlinePlus says tell your doctor if you take disulfiram or stopped it within the past 2 weeks - the combination is ruled out. Both drugs interact with alcohol metabolism; stacked, the reaction risk multiplies. This is exactly the kind of history your prescriber needs before the first dose.
Can I breastfeed while taking metronidazole?
It passes into milk, so there is a plan for it. MedlinePlus says your doctor may tell you to pump and discard your milk while taking metronidazole and for 48 hours after the final dose - ask for the feeding plan before you start, not after. If you become pregnant during treatment, call your doctor; the first 3 months of pregnancy generally rule it out.
Why do I have to finish metronidazole when I feel fine?
The infection can outlast the symptoms. MedlinePlus says stopping too soon or skipping doses can leave the infection incompletely treated, and the surviving bacteria can resist antibiotics later. This is doubly true for the infections metronidazole treats - an undertreated one comes back, sometimes to a partner too. Finish every tablet.
What happens if I miss a dose of metronidazole?
Take it as soon as you remember, unless it is almost time for the next one - then skip it and never double up. If you were prescribed the one-time dose and you are unsure whether it went down, call your pharmacist before taking another - the single-dose regimen has no catch-up rule.